This invention concerns improvements in or relating to optical apparatus and relates more particularly to optical apparatus for biocular viewing.
Optical apparatus for biocular viewing is used, for example, in low light level or night vision equipment by which an observer can view with both eyes a magnified image of an object which is usually the face-plate of an image intensifier tube. Light from a scene or object at a low brightness level is focussed by an objective lens system on to the image intensifier tube which, by an amplifying electro-optic system, displays on its face-plate an image of the scene or object of sufficient brightness for viewing by a human observer. Light from the displayed image travels via some form of magnifier to the observer's two eyes. In some applications of biocular viewing apparatus the object can be, instead of an image intensifier tube, a cathode ray tube.
Biocular apparatus which provides both eyes with a view of a single image intensifier or cathode ray tube had distinct cost and weight advantages over binocular apparatus in which two separate tubes and associated optics are provided, one for each eye. However, in biocular apparatus the optics have to meet the requirement of providing satisfactory imagery for the two spaced eye positions from a single object.
Two eye vision of a single object can be achieved by the use of a biocular magnifying lens, i.e. a lens specially designed to have an exit pupil sufficiently large for use with both eyes, and examples of such lenses are described in British Pat. Nos. 1,389,564 and 1,537,276. It has also been proposed in British patent specification No. 1,433,333 to cement the object to a solid body having planar and concave reflecting surfaces which form a final image at infinity, the extent and/or disposition of the reflecting surfaces and the extent of the solid medium being sufficient to permit biocular viewing of the final image by an observer having his eyes respectively before eye windows or output faces of the solid body. There have also been proposals for the use of a collimating lens system plus means for providing each eye with vision through a portion of the collimator aperture. In particular British patent specifications Nos. 1,506,614 and 1,556,511 effectively describes multi-reflection pupil expansion, British patent specification No. 1,542,124 describes a prism arrangement, British patent specifications No. 1,076,639 and GB 2020943 A describes pupil transfer by telescopic means, and British patent specificaion No. 923,925 describes an arrangement with telescopes and prisms.
It is sometimes highly desirable that the biocular viewing apparatus should be mounted actually on the observer's head in the manner of goggles. This has the advantage relative to a fixed mounting of permitting head movement while maintaining a view through the apparatus and relative to hand holding of leaving the observer's hands free to perform other functions. Necessary requirements for head mounting are small size and low weight. These same requirements are, however, also desirable for modes other than head mounting, especially for man portable equipments.
The advent of relatively small and lightweight image intensifier and cathode ray tubes has greatly assisted in reducing the size and weight of the equipment and reasonable portability and even head mounting has been achieved. Nevertheless, there is a continuing demand for reduction in size and weight allied with a desire for low cost while, of course, maintaining adequate optical performance.